The present invention relates to integrated-circuits and, in particular, to a circuit and a method for sensing a high voltage signal at a connection pin or signal pin of an integrated circuit.
Many integrated circuits require high-voltage sensor circuitry to detect the voltage levels at one or more connection pins. A supply-level voltage at a particular connection pin of a nonvolatile memory device may indicate, for example, that a read operation is to be performed. However, a voltage higher than supply-level at that same pin may indicate that another operation is to be performed. For example, on a flash EPROM device, a supply-level voltage applied to the programming-voltage pin indicates that reading operation is to be performed whereas a voltage of 12 V applied to the programming-voltage pin indicates that either a programming or an erasing operation is to be performed. In flash EPROMs, high voltage levels applied to various pins during manufacture are used, for example, to reconfigure the device, to replace defective columns with redundant columns, to disable sections, and to perform bitline short tests. Briefly, high-voltage sensor circuitry detects the high-level voltage input at a connection or signal pin and, in response, provides an output signal to other circuitry in the device to cause the device to enter a special operational mode (e.g., program, erase or test mode) other than the normal mode (e.g., read mode). If the high-voltage sensor circuitry does not function properly, a device such as a nonvolatile memory may be erroneously programmed, erased or stressed.
Normally, the output of high-voltage sensor circuitry provides a low-voltage (Vss or ground). However, if the input voltage to the circuitry is above a certain voltage level, the output switches to a high-voltage (Vcc, the power supply voltage). The switch to the high-voltage output occurs if the input voltage is above a specified reference-voltage level, usually the supply voltage Vcc plus a specified margin.
During the sequence of coupling the power supply voltage Vcc to the integrated circuit (power-up), or of de-coupling the power supply voltage from the integrated circuit (power-down), many prior-an high-voltage sensor circuits furnish an output signal erroneously indicating that a high-voltage level has been applied to device. That is because prior-art high-voltage sensor circuits are designed to detect high-input-voltage levels only during normal operation. Therefore, those prior-art circuits require that the power-up sequence be followed exactly to ensure that the high-voltage sensor circuit does not erroneously furnish an output signal indicating detection of a high-voltage level.
A typical prior-an high-voltage sensor circuit detects a high voltage at a level that is approximately equal to the supply voltage Vcc plus two P-channel threshold voltages. This two P-channel-threshold-voltage margin ascertains that the circuit does not consume any power when the input voltage to the circuit is less than the supply voltage Vcc plus the two-P-channel threshold-voltage margin. However, the output of the prior-art high-voltage sensor can erroneously furnish a high voltage signal, particularly during the power-up or power-down sequence. Assume, for example, that the prior-art high-voltage sensor circuit is designed to trigger at the nominal value of Vcc (5 V) plus two P-channel threshold voltages, each equal to about 1 V. However, if supply voltage Vcc is actually 3 V instead of the nominal 5 V (3 V is within tolerance for many integrated-circuit applications) any voltage greater that 5 V applied to that particular pin will cause the high-voltage sensor circuit to erroneously switch to an output indicating that a high voltage has been applied to the pin. And because the supply voltage is always 3 V for a moment of time during each of the power-up and power-down sequences, the device may be caused to enter an undesired operational mode during those sequences.